Hydraulic tensioners have been widely used to maintain proper tension, and suppress vibration, in engine timing chains, i.e., the endless chains used to transmit rotation from an engine crankshaft to one or more valve-operating camshafts.
In a typical conventional hydraulic tensioner 500, as shown in 6A, 6B and 7, a plunger 520 fits slidably in a plunger-accommodating hole 511 formed in a housing 510. The plunger has a cylindrical hollow portion 521 with one end open, and accommodates a plunger-biasing spring 550, which urges the plunger 520 in a protruding direction.
A high pressure chamber R is formed by the plunger-accommodating hole 511 and the cylindrical hollow portion 521 of the plunger 520. A check valve unit 540 permits flow of oil into the high pressure oil chamber, but blocks reverse flow of oil. The check valve unit comprises a ball seat 541, a check ball 542 facing the ball seat 541, a ball-biasing spring 543, which urges the check ball 542 into engagement with the ball seat 541, and a retainer 544, which supports the ball-biasing spring 543.
When the conventional hydraulic tensioner 500 is set at a downward angle in an engine, as shown in FIGS. 6A and 6B, the high pressure chamber R of the hydraulic tensioner 500 becomes filled with oil while the engine is in operation, and proper tension is maintained in timing chain C. For further details of the typical conventional hydraulic tensioner 500, reference may be made to United States Patent Application publication no. 2002/0142871, published Oct. 3, 2002.
When the conventional hydraulic tensioner 500, is set at a downward angle, when the engine is stopped and left standing for a long period of time, oil in the high pressure oil chamber R leaks downward by gravity, through a slight gap between the plunger-accommodating hole 511 of the housing 510 and the outer circumferential surface of the plunger, as shown in FIG. 7. As the oil leaks, outside air A is drawn into the high pressure chamber R through the gap between the plunger and the wall of the plunger-accommodating hole. A reduction in pressure within chamber R can result in an acceleration of oil leakage because, as the pressure decreases, more outside air is drawn into the chamber R, and, as a result, oil can flow out of the chamber by gravity at a faster rate. As shown in FIG. 7, a relatively large portion of the high pressure chamber R can become filled with air A. The compressibility of the air in chamber R prevents the tensioner from exerting an adequate damping force, i.e., the force exerted on the chain by the plunger, the magnitude of which is determined by the rate of leakage of oil through the gap between the outer circumferential surface of plunger and the cylindrical wall of the plunger-accommodating hole. The impairment of the operation of the tensioner due to accumulation of air in the high pressure chamber R is particularly significant upon engine start-up after the engine has been stopped and left standing for a long period of time. When air accumulates in the high pressure oil chamber, backlash of the timing chain C cannot be suppressed and an abnormal sound is generated. Additionally, the timing chain C can be unexpectedly damaged. Another problem with the conventional hydraulic tensioner is that it consumes oil at a relatively high rate, and requires an oil pump having a high output.
Still another troublesome problem with the conventional hydraulic tensioner 500 was that, in manufacture, it was difficult to achieve a proper clearance between the cylindrical wall of the plunger-accommodating hole 511 and the outer circumferential surface of the plunger 520, as the clearance is affected by various factors including size and measurement errors, surface finishing, materials and the like.
Accordingly, an object of this invention is to solve the above-described problems, and to provide a downward angle-settable hydraulic tensioner in which the full hydraulic damping force is achieved upon starting of the engine, so that backlash of the timing chain is suppressed and the generation of abnormal sounds is prevented.